The present invention relates to a hollow medical needle for use in ultrasonic imaging and to a method of enhancing the visibility of such a needle to a Doppler ultrasound imager.
The theory of ultrasonic transmission in biopsy needles is discussed in ULTRASONICS Vol. 26, No. 1, 1988 pp 27 to 30.
U.S. Pat. No. 3,556,079 (Omizo) discloses a medical apparatus comprising a tubular needle within which is mounted an ultrasonic transducer which may be a transmitter, a receiver or a combined transmitter and receiver. This transducer is coupled to saline solution within the needle and transmits and/or detects ultrasound at considerable distances through body tissue at a frequency of e.g. 5 MHz which is subsequently reflected off e.g. flowing blood in a blood vessel and shifted in frequency according to the Doppler equation. If the transducer in the needle is a transmitter only, then an external ultrasound receiver is arranged to detect the reflected Doppler-shifted ultrasound which is demodulated to generate an audio signal whose amplitude is a maximum when the needle is directed at the blood vessel. However the above arrangement merely enables the position of the blood vessel or other target to be detected and does not enable the position of the needle to be detected. It could not, for example, be used to direct a needle towards a portion of diseased static tissue. Furthermore it is not an imaging system.
Ultrasound imaging utilises the principle of sound reflectivity in order to form images of body organs. These images are displayed on the monitor in grey-scale. Some ultrasound machines also incorporate the principle of Doppler frequency shift which allows moving objects e.g. red blood cells in a blood vessel to be imaged. Such vessels can then be assigned a colour according to the direction of movement within them, and they appear in colour against the grey-scale background of their environs.
It is often desirable to form such an image during treatment with an aspiration or a biopsy needle and it has been found difficult to produce a clearly defined image of the needle by this technique. It has been proposed to apply a transverse vibration to the needle in order to overcome this problem but this does not result in a clear image of the needle tip.
GB-A-2,157,828 discloses an ultrasonic imaging apparatus comprising an inner needle having a miniature ultrasonic transducer mounted at its tip and an outer tubular puncture needle surrounding and supporting the inner needle. An ultrasonic imager generates an ultrasonic beam which impinges on the miniature transducer and the resulting electric output signal is either used to trigger a transponder which causes the transducer to emit a predetermined ultrasonic signal which is detectable and locatable by the imager or to enable circuitry in the imager to detect the position of the needle from the time interval between emission and detection of the ultrasonic beam. From this information the position of the needle can be superimposed on the image. However the above arrangement is complex and expensive and in general the needle can be used only with one design of imager.
Further background information is provided in U.S. Pat. No. 5,131,394, EP-A-397,960, EP-A-83,973 (which teaches at p. 7 para 1 the desirability of contact between the stylet and the needle) and WO-A-82/03160, which however lies outside the technical field of the present invention.
EP-A-453,251 discloses a biopsy needle having a solenoid coil coupled to a core which is mounted directly on the proximal end of the stylet and arranged to reciprocate the stylet longitudinally at a frequency of e.g. up to 100 Hz in order to render the needle tip visible to Doppler ultrasound. The amplitude of vibration is such that the tip of the stylet oscillates between a position in which it is flush with the opening of the cannula and a position in which it is retracted about 1 mm into the cannula.
However the above arrangement in which the vibratory transducer is directly coupled to the stylet has the disadvantage that movement of the stylet inevitably causes motion of similar amplitude and frequency in the cannula, with the result that the motion is not confined to the needle tip and the Colour Doppler imager assigns a flare of colour to the entire needle. Furthermore the weight of the solenoid makes the needle harder to manipulate, an important point when one considers the degree of accuracy needed to perform certain biopsies.